We aim to identify a CD4+ T cell signature for allergic diseases resulting from a comprehensive understanding of the mechanisms associated with the pathogenesis of allergic disease and peripheral tolerance to allergens. Investigations will be performeTd both in allergic individuals with heterogeneous allergic responses and in non- atopic individuals. The proposed work will include the use of emerging technologies including pMHCII tetramer staining and CD154 assay to track the allergen-specific CD4+ T cells ex vivo, together with 12-parameters flow cytometry and microarray transcriptional profiling to characterize specific immune responses at the single cell level. Using these technologies we will primarily investigate mechanisms associated with allergic responses in allergic subjects. This information is pertinent for understanding the biology of allergen-specific TH2 cells and will provide an opportunity to define a signature that reflects an underlying allergic disease process. Understanding the nature of allergen-specific CD4+ T cell responses in healthy individuals is also critical to improve current allergy vaccines, with the assumption that natural responses are protective against allergic inflammation, and thus that allergen-specific immunotherapy should recapitulate such immune responses. We will finally determine the extent to which this allergic T cell signature can be used as a surrogate biomarker to evaluate the efficacy of ASIT. This investigation will be performed in the context of an Immune Tolerance Network funded randomized double-blind, double dummy, controlled clinical trial (SLIT vs. placebo, SCIT vs. placebo). We hypothesize that: 1. Allergen-specific TH2 cells represent a stable and distinct subset of TH2 cells (denoted TH2A subset) defined by CD161 expression and lack of CD27. 2. TH2A cell population is absent in non-atopic individuals. 3. TH2A cells are functionally and molecularly distinct from conventional TH2 cells. 4. Allergen-specific TH2 cells possess a survival burden during SIT. 5. CD27+ allergen-specific T cells from allergic individuals reflect some classic features of the protective immune response to allergens. 6. TH2A cells can be used as a clinically meaningful biomarker in allergies.